Background

The pursuit of the most authentic audio quality drives continuous innovation in the HiFi audio field. To convert digital audio to analog sound effectively, higher data transmission rates are necessary to accurately reproduce realistic analog sound. High-end Hi-Res playback devices must support higher sampling rates, such as 384KHz and 768KHz, to provide richer spectral information, a wider dynamic range, a higher signal-to-noise ratio (SNR), and lower total harmonic distortion (THD).

Many consumer devices, such as smartphones and PCs, lack integrated Hi-Res sound cards. Therefore, using USB Audio for external sound card expansion is the best option for HiFi audio playback. The HiFi USB Audio solution employs the USB UAC2.0 protocol for asynchronous high-definition audio transmission, decoding audio up to 768KHz PCM and DSD 512. This ensures precise audio resolution and meets the high-fidelity, lossless playback demands of HiFi audio enthusiasts.

Overview

The USB Audio HiFi sound card solution offers a high-performance option for HiFi playback, integrating XMOS's proprietary XCORE architecture, high-resolution and flexible I/O, precise clock synchronization, and high-fidelity digital audio transmission. It features:

• Adoption of the UAC 2.0 (USB Audio Class 2.0) protocol and asynchronous transfer mode to ensure high-bandwidth USB transmission of high-definition audio data. It is compatible with major operating systems such as Windows, Android, and Linux.
• High-resolution and flexible I/O that meets the requirements of high-speed I2S/DSD and SPDIF audio interfaces, accommodating various audio and multi-channel needs.

This solution requires high-quality external audio clock sources in the peripheral circuit, allowing users to choose low-jitter clock sources (e.g., with low PPM and minimal temperature drift). The solution uses Asynchronous clock mode within the USB protocol, where the clock source synchronized between the host (e.g., PC) and the playback device is a locally low-jitter active clock on the device, ensuring clock accuracy and bit-perfect audio data transmission.

Users can employ high-end external DAC chips, selecting DAC types with excellent high sampling rates, SNR, THD, and high channel separation. Users have maximum flexibility in power design, tonal preference design, and Operational Amplifier (op-amp) selection. With the bit-perfect foundation of the XMOS HiFi USB Audio solution, users can fully optimize the analog audio design required for HiFi audio.

Features

USB Protocol

• USB 2.0 (Full-speed and High-speed)
• USB Audio Class 1.0
• USB Audio Class 2.0
• USB Firmware Upgrade (DFU)
• UAC 2.0 protocol, supports ASIO, and is compatible with various operating systems such as Windows, Linux, Android, macOS, and iOS.

Audio Interfaces

• I2S
• S/PDIF out
• Direct Stream Digital (DSD)

Audio Sampling Rates

• PCM: 44.1kHz, 48kHz, 88.2kHz, 96kHz, 176.4kHz, 192kHz, 352.8kHz, 384kHz, 705.6kHz, 768KHz
• DSD: DSD 64, DSD 128, DSD 256, DSD 512
• DOP: DOP 64, DOP 128

Solution Diagram

The XMOS USB Audio HiFi solution can leverage various XMOS chip platforms, such as XU208-128-QF48, XU216-512-TQ128, and the latest XU316-1024-QF60B-PP24. The XMOS chip platform in the USB Audio solution ensures a complete UAC protocol, audio interface drivers, low-jitter clock quality control, and bit-perfect quality control, serving as a robust bridge for USB to I2S/DSD output.

Key components:

• XU316-1024-QF60B-PP24 requires 3.3V, 1.8V, and 0.9V power supplies.
• Optionally, an external clock (Oscillator) can be used to provide I2S MCLK to the DAC.
• Requires an external 16Mbit flash for storing XU316 firmware.
• Requires external high-end DAC, op-amp, and other analog devices.

Software Framework

The XMOS USB Audio solution utilizes XMOS's unique XCORE multicore architecture. This solution can be implemented on various XMOS chip platforms. Using the XU316-1024-QF60B-PP24 as an example, the implementation principles of this solution are elaborated. Users are encouraged to refer to the documentation for a deeper understanding of this architecture.

• XMOS USB Audio White Paper (Fundamentals-of-USB-Audio_1.0.pdf)
• Software Development Guide (sw_usb_audio).pdf

It is important to note that at least five cores are required to run the XMOS USB Audio solution on the XMOS platform.

Application Scenarios

The XMOS USB Audio solution is suitable for the following products: